1. Field of the Invention
The present invention generally relates to motor vehicle power train assemblies. More specifically, the present invention relates to an all wheel drive power train assembly.
2. Description of the Prior Art
Historically, automobiles in the United States have primarily utilized rear wheel drive power delivery schemes. In adapting these rear wheel drive schemes into four wheel drive applications, a transfer case was, and often still is, positioned at the output of the transmission assembly. When engaged, the transfer case diverts a portion of the power coming from the transmission assembly from the rear wheels to the front wheels.
Today, a significant portion of new automobiles in the United States, and perhaps the world, are front wheel drive based vehicles. In a typical front wheel drive vehicle, typically both the engine and the transmission assembly are transversely oriented relative to the vehicle. By positioning the engine and transmission assembly transversely in the vehicle, a more direct coupling of the transmission assembly to the vehicle""s transaxle and front wheels is achieved. In doing so, the final drive unit (where the last torque multiplication takes place) and the front wheel differential are often incorporated directly into the transmission assembly itself.
With front wheel drive vehicles themselves becoming a mature market, a recent trend in the automobile industry has been to adapt front wheel drive schemes for all or four wheel drive applications. This is accomplished by providing a power transfer unit that diverts a portion of the power from the front wheels to a rear wheel drive shaft and, subsequently, the rear wheels.
Seen in FIGS. 1a and 1b is a typical prior art power transfer unit 10. The power transfer unit 10 includes a housing 12 within which is located a gear set 14 comprised of a parallel gear set 16 and a non-parallel gear set 18. The parallel gear set 16 includes a cylindrical extension 20 that operates as its input and this input 20 is coupled to the transversely oriented output 22 of the transmission assembly by way of a splined engagement 24. The cylindrical extension 20 is off of a gear wheel 26 or may be a sleeve to which the gear wheel 26 mounts. From the gear wheel 26, power is transferred through a second and third gear wheel, respectively 32 and 34. These gear wheels 32 and 34 are each supported on bearings 36 for rotation about axes 38 and 40 that are parallel to the rotational axis 28 of the first gear wheel 26.
The non-parallel gear set 18 includes a bevel ring gear 44 that is mounted to a shaft or sleeve 42 onto which the third wheel gear 34 is mounted or formed therewith. The bevel ring gear 44 engages a bevel pinion gear 46 mounted to another shaft 48 whose axis is generally perpendicular (and therefore non-parallel) to that of shaft 42. Mounted to an opposing end of the shaft 48 is an output member 50, illustrated as including a flange 52 and appropriately located bolt openings 54. The latter features enable the output member 50 to be bolted to a rear drive shaft (not shown).
As seen in FIG. 1a, the line 56 along which the gear wheels 26, 32 and 34 of the parallel gear set 16 engage with one another generally corresponds with the axis about which the output member 50 rotates. When locating of the power transfer unit 10 relative to the output of the transmission 22, this line 56 may be offset from the centerline of the vehicle. Additionally and as seen in FIG. 1b, the axis 28 of input into the power transfer unit 10, is offset, vertically or elevationally in the vehicle, relative to the output axis 58 about which the output member 50 rotates. This xe2x80x9cdropxe2x80x9d or height decrease from the transmission assembly output 22 to the output member 50 results from the relative positioning of the first, second and third helical gears 26, 32 and 34 of the parallel gear set 16, in conjunction with the non-parallel gear set 18. The non-parallel gear set 18 is a hypoid beveled gear set where the axis of rotation 40 of the bevel ring gear 44 does not intersect the axis 58 of rotation of the bevel gear 46.
Located within the transmission assembly, and before the output 22 thereof, is a final drive unit (illustrated schematically as box 57). The final drive unit 57 performs the last torque multiplication in the power train and is configured in any one of a number of conventional constructions well known in the art. One such construction is a planetary gear set with the input to the final drive unit 57 being through a sun gear while the output of the final drive assembly 57 is through a ring gear. The ring gear may be directly coupled to the transmission output 22 and therefore a large amount of torque is delivered to the power transfer unit 10.
Since the power transfer unit 10 receives power after the final drive assembly 57, the components of the power transfer unit 10 must be robust enough for high torque loads to pass therethrough. This adds size, weight and expense to the power transfer unit 10.
In view of the above and other limitations on the prior art, it is an object of the present invention to provide a drive train construction that permits a simpler design of various components, a decrease in the weight of the drive train, a compact construction and a limited amount of power transferred to the rear wheels.
In overcoming the above and other limitations on the prior art, a power train embodying the principles of the present invention is provided with intermediate output of a transmission that delivers power from the engine to a center differential (a planetary gear differential), which splits and transfers torque to the front wheels and the rear wheels of the vehicle. Through utilization of the power flow scheme of the present invention, it will readily be seen that the power transfer unit can be utilized on the transaxle before the final drive assembly. This allows the power transfer unit""s components to be smaller, lighter and simpler in design.
In transferring torque to the front wheels, the sun gear of the center differential causes rotation of a sleeve, which in turn is the input into a planetary gear, final drive unit. The final drive unit multiplies the torque and transfers power to the front differential, which is illustrated as a bevel differential. Thereafter power is provided to the half shafts and subsequently to the front wheels.
In transferring torque to the rear wheels, another sleeve, coupled to the carrier of the center differential, operates as the input member for the power transfer unit and is splined to the input gear wheel of the parallel gear set. A gear wheel, chain, belt or other feature connects the input gear wheel to an output gear wheel of the parallel gear set which is integrally formed with a sleeve or splined to a shaft. The sleeve or shaft operates as the output of the parallel gear set and is connected to the input of a non-parallel gear set.
In the non-parallel gear set, power is generally transferred via the sleeve or shaft to a set of bevel gears and specifically from an input or first axis to a second axis that is non-parallel to the input axis. This transfer of power is effectuated by a first bevel gear mounted or integrally formed on an opposite end of the sleeve or shaft to which the output gear wheel is mounted. A second bevel gear (engaging the first bevel gear) is mounted to a shaft, or integrally formed with the shaft, and operates as the output of the non-parallel gear set. Finally, an output member coupled to the shaft transfers power to the rear wheel drive shaft from the non-parallel gear set.
Simultaneous with the transfer of power to the power transfer unit and the rear wheels, power from the center differential is transferred in parallel to a biasing or torque limiting device. This device is coupled to the input gear wheel of the power transfer unit and operates so as to limit the amount of torque transferred therethrough to the rear wheels.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art, to which the present invention relates, from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.